After the spring is compressed (or stretched), at the level of "conservation of mass and energy", the spring will definitely become heavier and satisfy: the potential energy of the spring is Ek=Δmc^2, but this kind of change is very small. The "mass-energy equation" points out that any change in the energy of a closed system will lead to a change in the relativistic quality, so that mass-energy conservation can be satisfied.
The microscopic level of spring compression (stretching): the molecular potential energy between spring molecules is increased. If the spring is made of iron, that is, the average distance between iron atoms decreases (increases), and the energy input from the outside is converted into The potential energy between the iron atoms, which will inevitably lead to an increase in the relativistic quality of the iron atom with increased potential energy, so the overall relativistic quality of the spring is also improved. This will subvert the law of conservation of mass in middle school a bit. That is because the law of conservation of mass is established without considering the theory of relativity. In the system of relativity, it is revised to "conservation of mass and energy".
If we compress a spring, we must input energy to the spring from the outside world. This part of energy will naturally lead to the improvement of the quality of relativistic theory. In chemical changes, the quality of each atom in different molecules is slightly different. The difference is caused by different chemical bonds, and chemical bonds store chemical energy. The essence of chemical energy in the theory of relativity is the quality of loss.
